What is it you want to do? Infantry? Yeomanry? REMF? Or perhaps the Honourable Artillery Company? Joking aside, go for it. It will give you a taste of the Army and perhaps give you an appetite to do it full time.

TA won't mind about exam results as the army conducts their own intelligence tests called BARB tests. These will check your reading, maths and reasoning ect. Chances are your unit unless you are going for a commission won't ask for proof.